Hydraulic brake systems for motor vehicles are commonly dual systems in which the front brake circuit is hydraulically isolated from the rear brake circuit so that a failure in one of the circuits does not impede operation of the other circuit. It has been recognized that, due to weight transfer of the vehicle from the rear wheels to the front wheels during braking, it is desirable to provide a lower pressure to the rear wheel brakes than is provided to the front wheel brakes. To provide this function, a fluid pressure proportioning valve is sometimes placed in the rear brake circuit between the master cylinder and the slave cylinders of the rear wheel brakes to maintain increases in the rear wheel brake pressure proportional to but less than increases in the front wheel brake pressure. One such prior art proportioning valve is disclosed in U.S. Pat. No. 3,623,776 to Ellis M. Wellman, and another is disclosed in U.S. Pat. No. 3,667,810 to Richard J. Silagy.
It has also been recognized in the art that, under certain conditions, it is desirable to equalize the rear brake pressure with the front brake pressure under very severe braking conditions. To perform this function, the art has provided various blend back proportioning valves which reduce the fluid pressure increases in the rear brake pressure over one range of pressures and which equalize front and rear brake pressures under very severe braking conditions.